During scheduled and some forced outages of electric utility steam driven electric generators, one of the major concerns is the condition of the stator coils. Many tests are performed to quantify stator integrity. One of the most time consuming of these tests has been the test of stator wedge tightness since it has required removal of the rotor to gain access to the stator bore area, specifically the tooth tip area where the wedges are located. Removal of the rotor requires two to three days alone. The accepted industry method of testing stator wedge tightness is for a technician to "tap" the wedge, feel the resulting vibration, and listen to the sound. A loose wedge will vibrate more than a tight one, and can be felt with the fingers. In addition, a loose wedge will emit a characteristic hollow sound, which the experienced technician quickly learns to recognize as a loose wedge.
It is very important that wedge tightness be carefully ascertained and corrected if deficient because the tightness of the stator wedge is the only structural element that prevents stator coil vibration due to the combined effects of magnetic and mechanical loading. Field experience has shown that failure to hold the stator coil stationary in the stator slot permits ever increasing levels of vibration leading to deterioration and finally failure of the stator mica insulation and, in many instances, grounding or "flashover" of the coils. When this occurs, the owner-operator of the unit is faced with a time-consuming and expensive rewinding process. For these reasons, stator wedge tightness is of interest during routine outages, and not just when the rotor is removed.
One of the difficulties in testing wedge tightness without removal of the rotor is that there is as little as 3.81 cm (1 5 in.) of clearance between the stator bore and the retainer ring through which apparatus may be inserted to inspect the wedges distributed along the length of the stator. Another difficulty is that the wedges are made of non-conductive, non-magnetically permeable material such as, for example, fiberglass coated with Kevlar which is, compared to other materials such as steel, an absorbent of mechanical energy so that the techniques available for measuring tightness are limited. An additional difficulty, especially in the case of an impact tester, is that the stator coils extend radially outward about a horizontal axis such that the effect of gravity on the impactor varies with the angular position of the stator wedge being tested.
Commonly owned U.S. Pat. No. 4,889,000 discloses a low profile remotely controlled carriage for insertion into the gap between the rotor and stator of an electric generator for performing inspections. The carriage is positioned over a wedge with the aid of a miniaturized television camera. A solenoid when energized strikes the wedge and a microphone records the acoustic response. It has been found, however, that it is desirable to apply a larger and more repeatable impact force to the wedge than can be developed by a solenoid. It has also been found that it is difficult to assess with a computer the acoustic response recorded by the microphone.
The inspection apparatus of U.S. Pat. No. 4,889,000 also includes an Electro-magnetic Core Imperfection Detector (EL-CID) tester which is used to assess the condition of the insulation between stator laminations. Commonly owned U.S. Pat. No. 4,803,563 also discloses an EL-CID tester mounted on a carriage inserted between the rotor and stator of an electric generator for inspecting the insulation between the stator laminations. The carriage in U.S. Pat. No. 4,803,563 is held in place against the stator by permanent magnets embedded in the carriage chassis.
Other attempts have been made to quantify the "tap, listen and feel" process for testing stator wedge tightness. A mechanical impedance probe has been developed which is based upon the recognition that during a resonance sweep, a tight wedge will resonate (shift phase) at a slightly higher frequency than a loose one. This method does not discriminate between difference degrees of looseness nor does the apparatus have sufficient power to resonate wedges of the size and style used with the larger steam driven units. In addition, the unit is too large to fit into the rotor stator air gap.
Another type of apparatus for measuring wedge tightness uses a force measurement system. The theory of operation is that when an impact force is applied to a stator wedge, the hammer will maintain contact with a loose wedge for a longer interval before recoiling than with the same wedge in a tight condition. This has been confirmed, however, the sensitivity of the test does not permit clear discrimination between the tight and loose conditions. Furthermore, a version of such a device small enough for use in the air gap of the generator has not been developed.
Commonly owned U.S. Pat. No. 4,962,660 which issued on Oct. 16, 1990 provides an apparatus which can determine wedge tightness without removal of the rotor. What is needed is an improved system which can provide a direct measurement of wedge displacement in conjunction with the force required to cause that displacement.